The present invention relates to electronic control systems aboard aircraft. In particular, the invention relates to electronic control system architecture for air management systems aboard aircraft.
An aircraft air management system supplies bleed air to a variety of aircraft air management sub-systems, such as an environmental control system to maintain aircraft cabin air pressures and temperatures within a target range for the safety and comfort of aircraft passengers, anti-icing systems, inert gas systems, air-driven pumps, etc. This is done through the use of compressed air taken from two compressor stages (bleed air) of an engine propelling the aircraft. A control valve operates in response to electronic control signals from the air management system to control bleed output pressure. Air pressure in the bleed output pressure line is measured by at least one pressure sensor which provides this information to the air management system. The air management system uses the air pressure information to command the control valve to provide the desired bleed air pressure to the air management system. This is referred to as the bleed air control loop.
Downstream from the bleed air control loop are the air management sub-systems that draw airflow from the outlet of the bleed system. Each of the sub-systems has its own control valve to adjust a control parameter (sub-system parameter), for example, a pressure or a flow, in a particular sub-system, along with a corresponding sensor to measure the control parameter. As with bleed air, the air management system also maintains control loops for each of the sub-systems, receiving sensor measurements and generating control commands for the control valves. However, because the downstream sub-system control elements (control valve, sensor) are in series with the upstream bleed system control elements, the downstream sub-system control loops are pneumatically coupled to the upstream bleed control loop. That is, a control change in a sub-system valve position impacts not only the sub-system sensor, as it should, but also the upstream bleed output pressure sensor. This triggers a control response in the bleed control loop resulting in a change in the bleed control valve. Similarly, because the control loops are coupled, a control change in the bleed control valve impacts not only the bleed output pressure sensor, but also the downstream sub-system sensor, triggering a control response in the downstream sub-system control loop resulting in an unintended change in the sub-system valve position. Thus, a single change can oscillate back and forth between coupled control loops, creating control instabilities resulting in rapid, but unnecessary, control changes. This coupling creates an unnecessary burden on the air management system electronics and causes the bleed and sub-system control valves to wear out much faster than without the control instabilities caused by the coupled control loops.
One solution is to operate coupled control loops of concern at very different rates. For example, if a bleed control loop is operated at a very fast rate and downstream sub-system control loop is operated at a slower rate, all the sub-system control loop sees a very stable pressure from the bleed system. Interaction between the loops is minimized because of the different time scale of the control loops. However, this constrains the choice of components and ultimate performance of the air management system. Making bleed control loops faster requires additional constraints be placed when designing the components which impact system cost and weight. Slowing down sub-system control loops reduces their responsiveness and performance of the sub-systems.